There are few sailors out there who use the under hand grip on the front hand (me included)
Here's a shot from way out west that shows under hand grip in a different sailing application.
For me personally it comes from learning to sail many many moons ago using the trusty Z boom.
I also find it puts less pressure on my wrist/elbow when sailing for an extended period.
A few crew who have developed carpal tunnel type injuries (tennis elbow) have been instructed by their respective physios to try the under hand grip to reduce stress/strain ?
I tend to jump, jybe and even wave sail this way
Big thanks to Mr Mindfactory for another shot from the vault.
Kev